Radiation detector systems, which involve radiation detectors for gamma rays, X-rays, and particle detection for neutrons, protons, and beta particles are often required to provide imaging capabilities in order to visualize the radiation interaction positions. These radiation detector systems comprise three principal subsystems: (i) radiation detectors materials, such as scintillators and solid state material (e.g., germanium, cadmium zinc telluride, etc. . . . ), (ii) a data acquisition system, and (iii) a computer or analogous device for data processing.
Data acquisition (DAQ) is the process of measuring an electrical or physical phenomenon such as voltage, current, temperature, pressure, or sound with a computer. A DAQ system consists of sensors, measurement hardware, and a computer with programmable software. As compared to traditional measurement systems, computer-based DAQ systems exploit the processing power, productivity, display, and connectivity capabilities of industry-standard computers providing a means for more powerful, flexible, and cost-effective measurements.
DAQ hardware acts as the interface between a computer and signals from the outside world. It primarily functions as a device to digitize incoming analog signals so that a computer can interpret them. The three key components of a DAQ hardware device used for measuring a signal are (i) the signal conditioning circuitry, (ii) analog-to-digital converter (ADC), and (iii) computer bus. Many DAQ hardware devices also include other functions for automating measurement systems and processes. For example, digital-to-analog converters (DAC) for the output of analog signals, digital I/O lines for the input and output digital signals, and counter/timers for counting and generating digital pulses. The DAQ systems form the core of detector systems used in nuclear and high-energy physics and in nuclear medicine imaging detector systems.
Special electronic systems, known as analog-to-digital (ADC) systems, are utilized to digitize analog electrical pulses that arise from radiation detectors that are solid state or scintillator based. Both in physics research and nuclear medicine, photomultiplier tubes (PMT) are a type of detector element that can generate high speed electrical pulses which need to be digitized in order to make a measurement. For instance, the Hamamatsu H8500 position sensitive photomultiplier tube (PSPMT) has 64 anode pads which, when the PSPMT is optically coupled to a scintillator, are used to convert gamma ray events into electrical charge. The 64 anode charge outputs can be combined by channel read out electronics into a 2x+2y coordinate map with four (4) voltage pulse outputs.
An ADC instrumentation system would be used to digitally capture the 4 outputs, process the data, and send the results to further electronics for processing. The ADC process could be a “total charge” digitization in which the total charge represented by the integration of the charge pulse is digitized or it could be the actual digitization of the charge pulse “shape.” The charge integrated through the digitization process is referred to as “charge ADC.” When the pulse shape is digitized this is often referred to as a “flash ADC.”
Typically, one data acquisition channel can only handle the conversion of a single continuing stream of analog pulses. Multi-channel DAQ systems—typically more than 4 channels—are needed to convert analog pulse streams of a Gamma Camera for gamma ray interaction position computation for imaging purposes. In this case, a generic commercial DAQ system must be used. Large radiation detection systems with multiple Gamma Cameras may be handled by such a commercial system with channel numbers of 8, 16, 32, or 64. More medical applications can be found for large systems such as positron emission tomography (PET) or single-photon emission computed tomography (SPECT).
In large detection systems, the pulses which are members of a coincident event are likely to occur in distant locations, both spatially and electrically. In addition, with variations of cabling and components, the apparent time of the pulse arrival may be skewed relative to other pulses arising from the same nuclear emission. Determining the coincidence of pulses in such systems has been addressed historically by two methods: Direct timing coincidence detection which requires that all pulse arrival signals are routed to a single point where coincidence is determined, or by time tagging and post-processing, where data for all pulses are collected with additional synchronized time tag information and the valid coincidences are sorted out in a post processing step. The former method imposes a significant requirement on system cabling and electronics and the latter method imposes a large data transmission and storage requirement along with a significant post-processing step. These methods are suitable for many types of apparatus, but when systems are intended to be frequently reconfigured electrically and mechanically the first method become impractical, and when system bandwidth is limited and event rates are high the latter method loses its practicality.
The collection of data from several types of data acquisition systems and processing of that data to reconstruct useful data products, such as visual images or tomographic projections, relies on the detection and identification of time-coincident arrival of signals from pulses in multiple detectors that arise from specific types of nuclear emissions from sources. These valid coincidences must be differentiated from noise and accidental coincidences. Determining which of the many coincident and non-coincident pulses constitute valid coincident events, without requiring burdensome cabling or transmission and storage of voluminous data, has been a challenge for many coincident data acquisition systems. The system disclosed herein addresses the problem by performing the coincidence determination using time tagging in near real time with a minimum of data transmission required.
It is therefore desirable to have an efficient, versatile signal digitalization and processing system capable of being easily expanded and without the need for the transmission and storage of large amounts of data.